The conventional technique with regard to the axle driving unit housing therein a hydraulic transmission has been well known as disclosed in, for example, the Japanese Patent Laid-Open No. Hei 1-223023 and No. Hei 1-309821. The power transmission structure of a working vehicle, as disclosed in U.S. Pat. No. 4,809,489 and U.S. Pat. No. 3,047,085, is also well known.